Exhaust after-treatment systems receive and treat exhaust gas generated from an internal combustion engine. Typical exhaust after-treatment systems include various components configured to reduce the level of harmful exhaust emissions present in the exhaust gas. For example, some exhaust after-treatment systems for diesel powered internal combustion engines include various components, such as a diesel oxidation catalyst (DOC), particulate matter filter or diesel particulate filter (DPF), and an SCR catalyst. In some exhaust after-treatment systems, exhaust gas first passes through the diesel oxidation catalyst, then passes through the diesel particulate filter, and subsequently passes through the SCR catalyst.
Each of the DOC, DPF, and SCR catalyst components is configured to perform a particular exhaust emissions treatment operation on the exhaust gas passing through the components. Generally, the DOC reduces the amount of carbon monoxide and hydrocarbons present in the exhaust gas via oxidation techniques. The DPF filters harmful diesel particulate matter and soot present in the exhaust gas. Finally, the SCR catalyst reduces the amount of nitrogen oxides (NOx) present in the exhaust gas.
SCR catalyst systems utilize a reductant to reduce NOx in exhaust gas. Typical SCR systems include a reductant delivery system that includes a reductant source, pump, and delivery mechanism. The reductant source can be a container or tank storing a reductant, such as, for example, urea solution or ammonium formate solution. The pump supplies reductant from the source to the delivery mechanism via a reductant line. The delivery mechanism, which typically is a reductant injector, delivers the reductant into an exhaust gas stream upstream of an SCR catalyst. In automotive applications, the reductant typically is urea, which decomposes to produce ammonia. After reduction, the ammonia reacts with NOx in the presence of the SCR catalyst to reduce NOx to less harmful emissions, such as N2 and H2O.
For proper operation, the temperature of the reductant stored in the reductant storage tank and pumped through the reductant line between the tank and delivery mechanism must be maintained above the freezing point of the reductant solution. Emissions regulations require SCR systems to provide a temperature control system for heating the reductant when operating at low ambient temperatures.
Due to the high power needed for heating large amounts of reductant, conventional SCR systems typically have a reductant temperature control system that uses engine coolant to heat the reductant stored in the tank. In contrast, because the amount of reductant flowing through reductant lines is relatively small, less power is required to heat reductant lines. Accordingly, conventional systems may employ electrical heaters instead of coolant to heat reductant lines.
Electrical heaters, however, suffer from several limitations. For example, due to the difficulties in measuring the temperature of reductant inside a reductant line, electrical heaters require an ambient air temperature sensor as the primary input for proper operation. This is because the energy for electrical heaters is supplied by an adjustably controlled applied voltage or current. Because the temperature increase of the reductant is dependent upon the supplied electrical energy rather than absolute temperature, the ambient air temperature as detected by an ambient air sensor is required as a reference point from which a proper heating temperature, i.e., an upper heating limit, is established. Unfortunately, ambient air temperature sensors may be defective or become inaccurate over time based on limits and locations of ambient air temperature sensors, which may lead to an increased risk of overheating the reductant. In addition to an ambient air temperature sensor, electrical heaters require a separate controller, battery, and control relay, each of which may add to the energy, software, and hardware costs, as well as the bulk and complexity of the system.
Alternatively, in certain conventional systems, coolant from an engine is used to heat reductant in the reductant lines instead of an electrical heater. A dedicated coolant control valve is commonly used for facilitating the flow of coolant through the reductant line with one or more additional coolant control valves used to facilitate the flow of coolant through the reductant tank. The controls used in conventional coolant-based reductant heating systems do not control or modulate the coolant flow rate. Rather, the controls switch the control valves on to allow coolant flow or off to prevent coolant flow based on pre-determined timing schedules according to a sensed ambient temperature. Accordingly, like electrical heaters, conventional coolant heaters typically require an ambient temperature sensor for providing a reference point.
Further, due to the absence of a temperature sensor in the reductant line, another limitation of conventional systems is the inability to ensure the temperature of reductant in a reductant line is maintained above the freezing point of the reductant and below an upper threshold corresponding with the maximum temperature allowed in the reductant delivery system.